LI-7700

LI-7700 Open Path CH4 Analyzer
Fast Atmospheric Methane Measurements
Designed for the Eddy Covariance Method
Innovative Solutions
Remote Field Deployment • Low power (8 W during normal operation)
• Light weight (5.2 kg)
• Low maintenance
• Designed for long-term, unattended, continuous
operation
Eddy Covariance
• High precision (5 ppb RMS noise @ 10 Hz, 2000 ppb CH4)• High-speed data output (up to 40 Hz)
• Simplified data integration • Analog inputs for sonic
anemometer data Open Path
• I n-situ methane density measurement using laser
spectroscopy
• Enables low power system operation
• No time delays or attenuation from tubing
• No pump required
Designed for Field Use
• Radiation shield minimizes condensation on the upper
mirror
• Carbon fiber spars minimize thermal expansion of the
optical path
• Heaters keep mirrors near ambient temperature to
prevent condensation
washer
fluid
Self-Cleaning Lower Mirror
• Decreases maintenance
requirements
• Ensures more robust,
continuous data sets
• Cleaning is initiated at
user-specified time intervals
or a signal strength threshold
Go to /mirror to watch a
video of the LI-7700’s self-cleaning mirror
The LI-7700 Open Path CH4 Analyzer The LI-7700 makes in-situ measurement of
methane density with the resolution, speed,
and stability required for the eddy covariance
technique.
Previously, measuring methane flux was difficult
because of instrument limitations, including high
power demands, heavy pumps, and the need for
instrument enclosures. The LI-7700 overcomes
all of these limitations.
The LI-7700 is light weight, has low power
requirements, and provides high frequency
response. The LI-7700 is designed to measure
methane density at frequencies that are neces-
sary to quantify ecosystem methane flux.
Rice paddies, bogs, wetlands, and other methane-producing ecosystems are unforgiv-ing places for scientific instrumentation. The
LI-7700 Open Path CH4 Analyzer is designed for use in these remote environments.
Methane flux data has been collected with the LI-7700 in a variety of ecosystems under a wide range of environmental conditions - from Arctic tundra to mangrove forests. The LI-7700 includes numerous innovations that reduce the need for site visits, while enabling the collection of continuous, long-term data sets in remote locations.
The open path design eliminates the need for pumps and tubing. Unlike closed path systems, there are no pressure drops, tubing attenuation, or time delays with open path systems.
Why the LI-7700?
• High precision and accuracy
• Low power
• Long-term, low maintenance
operation
• Easily integrates into flux towers
• Simplified data integration
• Light weight
LI-COR recognizes field instruments require a durability that bench-top instruments often lack.
The LI-7700 is designed to make high quality measurements in extreme environments.
Measure CH4 Everywhere
Florida Everglades
Low Power… to go where you need to go.
Methane-emitting ecosystems tend to occur in
regions that lack easy access to AC power.
This complicates measurements of ecosystem meth-
ane flux. The LI-7700 is designed for DC (battery and
solar panel) operation in remote areas and extreme
environments. This provides you the freedom to take
the instruments wherever you want to measure, with-
out being constrained by power requirements.
Innovative Technology
Recent advances in laser technology and advanced engineering enabled LI-COR Biosciences to develop the LI-7700 Open Path CH4 Analyzer. The LI-7700 uses a single-mode tunable near-infrared laser source, operating at ambient temperature, which greatly reduces power consumption and eliminates the need for cryogenic cooling systems.
Using Wavelength Modulation Spectroscopy (WMS), the laser scans across a single feature in the 2ν3 absorption band of methane near 1.6 microns with high resolution and at a high repetition rate. The wavelength is modulated at sub-MHz frequency, virtually eliminating 1/f noise of the laser source and allowing detection of fractional absorption (smaller than 10-5). The LI-7700 demodulates the resulting signal at twice
the modulation frequency, and then compares it to a reference signal shape to deter-mine CH4 concentration. Pressure and temperature induced changes in line shape and population distribution, as well as changes in laser power and mirror reflectivity are compensated for using computational fitting algorithms. This ensures measurements remain accurate over a wide range of pressure and temperature conditions.
This combination of innovative technologies allows for an open path measurement with the speed and resolution required for eddy covariance measurements of methane. Wavelength Modulation Spectroscopy Wavelength Modulation Spectroscopy has distinct advantages over other spectro-scopic techniques used for trace gas measurements. It provides a strong signal with very low noise. This enables the use of a shorter optical path-length with fewer reflec-tions and reduces sensitivity to contamination of mirrors.
A multi-pass Herriott cell with off-axis alignment allows for a short physical path length
(0.5 m) but a 30 m total optical path length. Other laser spectroscopy techniques require optical paths up to several kilometers in length to make a comparable measure-ment.
The 0.5 m optical cell enables the methane density measurement with relatively few reflections. With fewer reflections, the mirrors in the optical path are less vulnerable to contamination than mirrors used in other spectroscopic instruments. Instruments that use Cavity Ring-Down Spectroscopy (CRDS) and Integrated Cavity Output Spectros-copy (ICOS) require mirrors with 99.99% or higher reflectivity.
The LI-7700 makes in-situ methane density measurements at ambient pressure. CRDS and ICOS rely upon vacuum pumps to draw down pressure in the optical cell which sharpens absorption lines.
Advantages of WMS
• Strong signal with low noise
• Reduced sensitivity to
contamination of mirrors
• Short physical path length
• Operation at ambient
pressure
The green laser shown at the
right is for the purpose of illus-
tration only. The LI-7700 uses a
near-infrared laser source that is
invisible to the human eye.
The LI-7700 was selected
by R&D Magazine as one
of the 100 most innova-
tive, technologically signifi-
cant products of 2010.
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A i r T e m p e r a t u r e , o C
C H 4 F l u x , m g m -2 h -1 C H 4 C o n c e n t r a t i o n , p p m
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C H c o n c e n t r a t i o n , p p m
Field Data
The figure below shows an example of methane flux data from a 23-day period over the Florida Ever-glades at a height of 3.15 m. Under high humidity conditions, the LI-7700 collected data continuously with very little user-intervention. Fluxes were obtained with the eddy covariance method using the LI-7700 and a sonic anemometer. The inset shows hourly averages of data collected over a single
24-hour period. In this example, reduced wind speeds in the evenings led to lower turbulent exchange and allowed methane to accumulate in the canopy. Each morning, the build-up was eliminated as wind speeds increased.
Publications which include the LI-7700 :
McDermitt, D., G. Burba, L. Xu, T. Anderson, A. Komissarov, J. Schedlbauer, D. Zona , W. Oechel, S. Oberbauer, G. Starr, and S. Hastings, 2010. A New Low-Power, Open Path Instrument for Measuring Methane Flux by Eddy Covariance. Applied Physics B: Lasers and Optics. Accepted.
Burba, G., L. Xu, J. Schedlbauer, D. Zona, T. Anderson, D.K. McDermitt, S. Oberbauer, W. Oechel, A. Komissarov, and B. Riensche, 2010. Eddy Covariance Measurements of Methane Flux at Remote Sites with New Low-Power Lightweight Fast Gas Analyzer. European Geosciences Union General Assembly, Vienna, Austria, 02 – 07 May. Pages 46-52 in the following publication:
Burba, G.G., and D.J. Anderson, 2010. A Brief Practical Guide to Eddy Covariance Flux Measure-ments: Principles and Workflow Examples for Scientific and Industrial Applications. LI-COR Biosci-ences, Lincoln, USA, Hardbound and Softbound Editions, 212 pp.Go online for more publications: /7700publications
Field data illustrate that the LI-7700 has the frequency
response required to sample the range of eddy frequencies that contribute to methane flux.
Co-spectra
Co-spectral analysis provides a powerful tool to evaluate the capability of the analyzer to detect gas fluctuations over a range of fre-quencies (Kaimal et al., 1972). It is important to validate frequency response of any new analyzer, open-path or closed-path, using actual field data, and compare it to co-located references such as a sonic anemometer or an open-path gas analyzer (e.g., LI-7500). Sometimes it is also useful to compare the actual co-spectral shapes to ideal ones from a model. However, the modeled shapes may not always account for specificities and imperfections of study site topography and airflows, while sonic anemometer tempera-ture flux co-spectra generally do account for these and are a practical reference of nearly-perfect frequency response.
The figures to the right show ensemble averages of normalized daytime co-spectra plotted versus non-dimensional frequency for contrasting ecosystems and measurement heights at a 10 Hz sampling rate.
In all cases, methane co-spectra behaved similarly to the co-spectra of CO 2, H 2O, and air temperature. This demonstrates that the LI-7700 configuration is suitable for measur-ing fluctuations in CH 4 concentration across the whole spectrum of frequencies that contributed to turbulent transport at these measurement heights.
All the co-spectra followed the Kaimal model (Kaimal et al., 1972).
Kaimal J., J. Wyngaard, U. Izui, and O. Cote. 1972. Spectral characteristics of surface-layer turbulence. Quart. J. Roy. Meteor. Soc, 98: 563-589.
Kaimal slope
Expected instrument slope at high frequency
N o r m a l i z e d c o -s p e c t r a , n C w x /W ’X ’
Non-dimensional frequency, f=nz/U
w’T’
w’CO 2
w’CH 4
TDLS
3-Gas Eddy Covariance System
For a multi-gas system, the LI-7550 Analyzer Interface Unit can be used to integrate an LI-7700, a CO 2/H 2O analyzer such as the LI-7500A Open Path CO 2/H 2O Analyzer or LI-7200 Enclosed CO 2/H 2O Analyzer and a sonic anemometer. When using the LI-7700 to calculate methane flux, it is critical that a high speed H 2O analyzer be used for water vapor correction.
The figure below shows methane, carbon dioxide, and latent heat fluxes measured with the eddy covariance technique over the Florida Everglades. These data are a subset of a 6-week data set collected during the winter of 2008-09, at 3.15 m above the canopy, using an LI-7500 Open Path CO 2/H 2O Analyzer and an LI-7700 Open Path CH 4 Analyzer. They show a net negative flux of CO 2, but positive CH 4 and LE fluxes for the measurement period, indicating that this ecosystem was a sink of carbon dioxide and a source of methane and water vapor.
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C O 2 F l u x (g /m 2/h )
C H 4 F l u x (m g /m 2/h )
L E (W /m 2)
Day of Year
-2.5-0.150.15-0.30.3Both the LI-7700 Open Path CH 4 Analyzer and the LI-7550 Analyzer Interface Unit provide four
general ±5 V inputs for analog data from any fast sonic anemometer. The LI-7700 also provides three type E thermocouple inputs for recording additional temperature data if desired. LI-COR sonic an-emometer cables (optional) allow you to easily integrate analog data from Gill WindMaster™/Pro or Campbell ® Scientific CSAT-3 sonic anemometers with the LI-7700 data stream. Weatherproof cable terminals provide simple and reliable electrical connections.
Greenhouse Gas Packages from LI-COR:
Greenhouse gas analyzer systems from LI-COR Biosciences facilitate the collection of eddy covari-ance data sets by logging CH 4, CO 2, and H 2O density data, along with wind speed, direction, and temperature data (u, v, w, and T s ) to a removable USB data storage device. Data can also be trans-ferred via Ethernet to an external data storage device or computer network.
LI-COR offers two Greenhouse Gas Packages which include the LI-7700 CH 4 Analyzer with either the LI-7500A Open Path CO 2/H 2O Analyzer or LI-7200 Enclosed CO 2/H 2O Analyzer. Sonic anemom-eters can also be purchased from LI-COR.
The LI-7700 Open Path CH 4 Analyzer - Eddy Covariance System Page 9
CH 4 Analyzer
Integrates easily into new or existing towers.
CO 2/H 2O Analyzer
LI-COR’s LI-7500A Open Path Analyzer (shown in the diagram) and LI-7200 Enclosed Analyzer make precise, accurate, and fast measurements. A CO 2/H 2O analyzer is a core component of a flux tower.
Sonic Anemometer
Measures wind speed and direction. LI-COR’s flexible
system allows for the integration of any sonic anemometer that has analog outputs.
The LI-7550 Analyzer Interface Unit
Used for data acquisition from the LI-7700, as well as data acquisition and control of the sonic anemometer and CO 2/H 2O analyzer. Data can be output through Ethernet, or stored inter-nally on a removable industrial-grade USB drive.
7700-101 Washer Assembly (included)
Supplies washer fluid for automated washing of lower mirror surface.
Solar Panels
With low power requirements (nominally 8W for LI-7700, 20W for LI-7700 and LI-7500A), LI-COR’s system can be easily
operated on solar panels and batteries.
Typical Eddy Covariance Configuration
LI-7700 Open Path CH4 Analyzer Specifications Resolution (RMS noise): 5 ppb @ 10 Hz and 2000 ppb CH4 Measurement Range:
0 to 25 ppm @ -25 °C, 0 to 40 ppm @ 50 °C
Accuracy at constant temperature:
typically < 1%, maximum < 2%
Drift from -25 °C to +45 °C: 0.05% per degree C Bandwidth: 1,2,5,10, or 20 Hz
Operating Pressure Range: 50 to 110 kPa
Operating Relative Humidity Range: 0 to 100% Operating Temperature Range: -25 to 50 °C
Data Communication: Ethernet (up to 40 Hz)
Detection method:
Wavelength Modulation Spectroscopy 2f detection
Power Requirements: 10.5 to 30 VDC
Power Consumption:
8 W nominal, 16 W during cleaning cycle
Dimensions:
Sensor: 14.33 cm dia (5.64 in), 82.8 cm height (32.6 in.) Optical Path: 0.5 m physical path (1.65 ft), 30 m measurement path (98.4 ft)
Weight: 5.2 kg (11.5 lbs)
User Interface: Windows® based software supports all setup, configuration, and calibration functions through Ethernet connection
Acknowledgements
LI-COR has licensed certain aspects of the LI-7700 that were developed with expertise from Southwest Sciences, Inc. (Santa Fe, NM). This project was funded in part by the US Department of Energy through a Small Business Innovation Research grant (DE-FG02-05ER84283).LI-7550 Analyzer Interface Unit Specifications
Data Storage: Removable USB Storage Device. 4 Gigabyte Provided (expandable with user supplied Industrial Grade USB Flash Drive)
Data Communication: Ethernet, Synchronous Devices for Measurement (SDM), RS-232 (57,600 baud), DAC Outputs (6) Inputs: Ethernet, 4 analog input channels
Operating Temperature Range: -25 °C to 50 °C
Power Requirements: 10.5 to 30 VDC
Power Consumption: 10 W nominal
Dimensions: 35 cm × 30 cm × 15 cm (13.8 in x 12 in x 6 in) external dimensions
Weight: 4.4 kg (10 lbs)
Specifications subject to change without notice.
Ordering Information
LI-7700 Open Path CH4 Analyzer:
Includes power and Ethernet cables, calibration fixture, washer assembly, mounting hardware, instruction manual, spares kit, carrying case, and software CD.
LI-7550 Analyzer Interface Unit (optional):
Provides data storage with a removable USB flash drive, and data output via Ethernet, SDM, RS-232, six DAC outputs for connecting to a data logger. Also includes four additional dif-ferential voltage inputs.
7550-101 Auxiliary Sensor Interface (optional):
Seven auxiliary inputs (four general ±5 V and three type E thermocouple channels).
LI-COR Biosciences
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11/2010 980-11617The LI-COR board of directors would like to take this opportunity to return thanks to God for His merciful providence in allowing LI-COR to develop and commercialize products, through the collective effort of dedicated employees, that enable the examination of the wonders of His works.
“Trust in the LORD with all your heart and do not lean on your own understanding. In all your ways acknowledge Him, and He will make your paths straight.”
—Proverbs 3:5,6。